1. Field of the Invention
The present invention relates to an improved process for the simultaneous separation of ethylene oxide and carbon dioxide from the gaseous mixtures obtained in the direct oxidation of ethylene with oxygen.
In the direct oxidation of ethylene to ethylene oxide two process methods are known one of which comprises the use of air and the other the use of oxygen as oxidizing agent. The oxidation is carried out over a silver catalyst using short residence times and at temperatures of from 150.degree. to 400.degree. C and pressures of from atmospheric pressure to about 30 atmospheres. In view of the danger of explosion a gaseous mixture is employed which contains relatively small quantities of ethylene. The conversion must not be too high in order to safeguard high selectivity. The gaseous mixture leavig the reactor generally contains 1 to 3% by volume of ethylene oxide. The mixture is cooled in a heat exchanger, then compressed and washed with water in a washer (absorber), in order to separate the ethylene oxide. The remaining ethylene is recycled to the process. As by-products of the direct oxidation carbon dioxide and water are obtained; see Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd edition, vol. 8 (1965), pages 534 to 542. Therefore one is faced with the problem of eliminating these by-products, in particular the carbon dioxide, or of keeping them to a desired concentration and avoiding their accumulation.
When air is used as oxidizing agent a part of the gaseous mixture leaving the reactor and being free of ethylene oxide is continuously blown off overhead in order to avoid gradual accumulation of inert gases, such as nitrogen. In this way, a fraction of the carbon dioxide corresponding to that continuously produced is eliminated from the reaction gases, with the result that accumulation of this by-product is avoided.
The use of oxygen as oxidizing agent has the advantage of eliminating or at least substantially reducing the blowing off of the washed out reaction gases still containing a small quantity of ethylene. However, in this case a treatment of the reaction gases is necessary in order to eliminate the carbon dioxide. After having been quenched the gaseous mixture leaving the reactor is first washed out with water in order to separate the ethylene oxide in the form of an aqueous solution. The gases which have not been absorbed are subsequently subjected to a treatment with a solution of an alkali metal carbonate or alkanolamine in order to separate the carbon dioxide. The residual gases which contain unreacted ethylene are then recycled to the reactor after previous addition of fresh ethylene and oxygen.
The process for the preparation of ethylene oxide using oxygen as oxidizing agent therefore has essentially the disadvantage of the double treatment of the reaction gases. Moreover, this process requires large quantities of water for the washing out of the reaction gases in the ethylene oxide washer. This leads to the necessity to recycle the lean aqueous solvent leaving the ethylene oxide stripper to the ethylene oxide washer. It is also known that this lean aqueous solvent still contains ethylene oxide in quantities of about 0.05% by weight, owing to the conditions under which the stripping phase is carried out industrially.
Thus, because of the low concentration of ethylene oxide in the reaction gases produced in the direct oxidation of ethylene and because of the presence of residual quantities of ethylene oxide in the lean aqueous solvent, this compound cannot be obtained with satisfactory recovery from the economic point of view under the temperature and pressure conditions under which the washing out of the reaction gases is carried out in practice. Thus, the large quantities of water required and the high heat capacity and heat of vaporization of the water lead to high heat consumptions in the stripping operation.